The present invention relates to an antenna device. It more particularly relates to a reverse F-shaped antenna formed on a surface of a printed board.
Various configurations of small-sized antenna systems suitable for use in mobile communication terminals and so on have been conventionally proposed. In such small-sized antenna systems, a reverse F-shaped antenna has been well known.
FIG. 1 is a diagram illustrating the configuration of a conventional reverse F-shaped antenna. The reverse F-shaped antenna is fed with power through an unbalanced circuit.
As shown in FIG. 1, the reverse F-shaped antenna comprises a radiating element (an exciting element) 5, a grounding element 6, and an RF connector 7 functioning as a feeding point for feeding power to the feeder 5c of the radiating element 5. The radiating element 5 includes a radiating conductor 5a, a short circuit conductor 5b, and a feeder 5c. A grounding terminal 7b of the RF connector 7 is connected to the grounding element 6.
Such reverse F-shaped antenna 1 is designed so that an impedance matching between a feeding line, such as coaxial cable 8, connected to the feeding point and the radiating element 5 may be obtained by changing the position of the feeding point.
However, the downsizing of wireless communication terminal bodies according to the recent population of mobile communication terminals and the downsizing of antenna modules are accelerating. This requires reverse F-shaped antennas in which the grounding element is incorporated into the antenna to be downsized. It is thus necessary to shorten the radiating element of the antenna.
Further, since radiating elements are printed on and formed in the same plane in the conventional reverse F-shaped antenna, electric current can flow only in the plane so that the antenna is less sensitive to waves polarized vertically relative to the plane.
In the conventional reverse F-shaped antenna, a resonance frequency is determined based on the length of the radiation member formed on the substrate. Thus, if any change in the frequency is required, it is necessary to redesign a separate substrate so that the length of the radiation member can be newly determined.
Alternatively, an array antenna by which dual frequencies can be shared using an inductor has also been proposed.
In this shared antenna, two feeding lines printed and formed on a surface and a back surface of the dielectric substrate, respectively, a pair of inner and outer radiating elements each of which is connected to each of the feeding lines, and an inductor positioned in the space between the inner and outer radiating elements to connect the elements are provided.
This allows the antenna to operate at a frequency f1, about a quarter of a wavelength of which is equal to the total length of the inductor and the inner and outer radiating elements. This also allows it to operate at a frequency f2, about a quarter of a wavelength of which is equal to the length of the inner radiating element, the frequency f2 being significantly higher than the frequency f1, by making the resonance frequency of a parallel circuit constituting a capacitor based on an effect of capacitance in the space and the inductor identical with the frequency f2.
Thus, in the above case, the antenna may operate at two different frequencies. However, since an inductor is positioned at the middle of the radiating elements, only a small amount of the downsizing effect of the radiating elements for the antenna can be realized.
What is needed is to provide an effectively downsized reverse F-shaped antenna having improved antenna properties.